Cholestasis (impairment of bile flow) is a common and often devastating manifestation of both acquired and inherited liver disease. However, the biology of normal bile formation and bile acid secretion are poorly understood, as is the pathophysiology of cholestasis. The goal of the proposed studies is to identify and functionally characterize the gene(s) responsible for two distinct, autosomal recessive, inherited cholestatic disorders: benign recurrent intrahepatic cholestasis (BRIC) and progressive familial intrahepatic cholestasis (PFIC), also commonly called Byler's syndrome after the originally described kindred. This proposal builds upon extensive preliminary studies, which include (a) development of a novel approach for localization of abnormal gene(s) in inbred populations that uses linkage disequilibrium (LD) between alleles of polymorphic DNA markers and the disease allele to identify conserved chromosomal segments harboring the disease gene(s), (b) use of this approach to map the gene(s) responsible for both BRIC and PFIC to a small (approximately 6 centiMorgan) region on chromosome 18, making it highly likely that the BRIC and PFIC genes are identical, and (c) development of probes for novel hepatic ATP-binding cassette proteins postulated to mediate hepatic bile acid secretion and to represent the abnormal gene in both disorders. The proposed studies will use a variety of genetic and physical mapping methods to identify the disease gene(s) and corresponding cDNA(s), including screening for chromosome rearrangements, to enable evaluation of candidate cDNAs from the smallest possible region. These cDNAs will then be used to screen patient genomic DNA for mutations and mRNA for altered patterns of expression. Following its definitive identification the function of the causative gene will be defined by examining the relationship between different mutations and disease phenotypes characterizing its function heterologous expression of the cDNA(s), and construction of a mouse model (i.e., knockout model) in which a mutant copy of the gene has been introduced. The proposed studies will provide important new insight into the biology of bile secretion and pathophysiology of cholestasis and are anticipated to lead to new approaches to the diagnosis and treatment for inherited and acquired liver diseases.